graph G {
  a[label="", shape=circle];
  b[label="", shape=circle];
  c[label="", shape=circle];
  {rank=same; b, c}
  d[label="merge [2] [3]", shape=rectangle];
  e[label="merge [1] [1]", shape=rectangle];
  f[label="merge [10] [0]", shape=rectangle];
  g[label="merge [14] [4]", shape=rectangle];
  {rank=same; d, e, f, g};
  a -- b;
  a -- c;
  b -- d;
  b -- e;
  c -- f;
  c -- g;
}
